Most common reasons check engine light comes on

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check engine light

Ignoring a check engine light now can lead to costly repairs later. Image: Robert Couse-Baker/Flickr/CC BY

The “check engine” warning light causes either panic or annoyance in most drivers. It can mean anything from a loose gas cap to incredibly expensive major repairs. Many drivers believe the system was intentionally made overly sensitive in order to trigger revenue-generating trips to the dealership. But why does the ubiquitous signal really illuminate?

A needed signal

Recent statistics point at a need for a device to signal drivers, caught up in their daily routines, to attend to timely automotive maintenance needs. According to the data firm R.L. Polk, we are driving our cars longer in the economic downturn. A decade ago, the average vehicle on U.S. highways was 8.8 years old. Today, that has risen to 10.6 years. reports that eight out of 10 cars on the road today are in need of repairs. CarMD says the average repair today runs $305.56, so the earlier caught, the easier on finances. But just how effective is the system?

[The average car in the US is 10 yrs old. Stand out, dare to buy a new car]

How does it work?

Since 1996, all U.S.-sold cars are required to have the On Board Diagnostics-II system installed. OBD-II is a standardized system for vehicle computers to detect and diagnose problems. When the “check engine” light illuminates, it means a problem has been detected and recorded in code form. By plugging a scan tool into the OBD-II port under the driver’s side dashboard, a mechanic can access that information. If the light stays on, it can indicate serious problems. If it blinks continuously, get thee to a mechanic now.

What follows are the five most common reasons a “check engine” light illuminates.

Oxygen sensor

The No. 1 dashboard-lighting defect is a faulty oxygen sensor. It measures the oxygen left unburned in the car’s exhaust system and determines the amount of fuel left in the tank. It can reduce mileage if it is sending incorrect information to the car’s computer. A simple home repair, if left unattended it can take big bites out of your wallet.

LifeHacker explains:

“Not replacing a broken oxygen sensor can eventually lead to a busted catalytic converter which can cost upwards of $2,000. Taking your car into a shop will cost you around $200, depending on the car. However, an oxygen sensor is easy to replace on many cars and is usually detailed in the owner’s manual. If you know where the sensor is, you only have to unclip the old sensor and replace it with a new one.”

Gas cap

The second most common reason the light comes on is if the fuel tank is not sealed at the top, allowing gas to evaporate and decreasing mileage. This fix is as simple as a quick twist, or a replacement cap if it is not sealing properly.

Catalytic converter

The catalytic converter reduces emissions from your vehicle and is a required piece of equipment in all U.S. models. Most catalytic converters contain platinum, so they can be really hard on the pocketbook to replace — generally about $2,000.

Mass air flow sensor

The mass air flow sensor determines how much fuel should be delivered to the engine. It it fails, the driver could experience loss of power or surges during acceleration. They generally cost about $400 to replace.

Spark plugs

If a vehicle’s spark plugs misfire, it can cause damage to the catalytic converter, as well as reducing fuel economy and engine power. With a little mechanical aptitude, this can be a home repair job for $10 worth of plugs. Or a mechanic can do the job for $300 or so.

Ignore at your peril

Every car is different and some “check engine” systems are more sensitive than others. But just putting a piece of duct tape over the light, or otherwise ignoring it, is risking increasingly costly damage to the vehicle. Drivers do so at their own peril.


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